The human spine is a complex structure designed to achieve a myriad of tasks, many of them of a complex kinematic nature. The spinal vertebrae allow the spine to flex in three axes of movement relative to the portion of the spine in motion. These axes include the horizontal (bending either forward/anterior or aft/posterior), roll (bending to either left or right side) and vertical (twisting of the shoulders relative to the pelvis).
In flexing about the horizontal axis into flexion (bending forward or anterior) and extension (bending backward or posterior), vertebrae of the spine must rotate about the horizontal axis to various degrees of rotation. The sum of all such movement about the horizontal axis produces the overall flexion or extension of the spine. For example, the vertebrae that make up the lumbar region of the human spine move through roughly an arc of 15° relative to its adjacent or neighboring vertebrae. Vertebrae of other regions of the human spine (e.g., the thoracic and cervical regions) have different ranges of movement. Thus, if one were to view the posterior edge of a healthy vertebrae, one would observe that the edge moves through an arc of some degree (e.g., of about 15° in flexion and about 5° in extension if in the lumbar region) centered about a center of rotation. During such rotation, the anterior (front) edges of neighboring vertebrae move closer together, while the posterior edges move farther apart, compressing the anterior of the spine. Similarly, during extension, the posterior edges of neighboring vertebrae move closer together while the anterior edges move farther apart thereby compressing the posterior of the spine. During flexion and extension, the vertebrae move in horizontal relationship to each other providing up to 2-3 mm of translation.
In a normal spine, the vertebrae also permit right and left lateral bending. Accordingly, right lateral bending indicates the ability of the spine to bend over to the right by compressing the right portions of the spine and reducing the spacing between the right edges of associated vertebrae. Similarly, left lateral bending indicates the ability of the spine to bend over to the left by compressing the left portions of the spine and reducing the spacing between the left edges of associated vertebrae. The side of the spine opposite that portion compressed is expanded, increasing the spacing between the edges of vertebrae comprising that portion of the spine. For example, the vertebrae that make up the lumbar region of the human spine rotate about an axis of roll, moving through an arc of around 100 relative to its neighbor vertebrae throughout right and left lateral bending.
Rotational movement about a vertical axis relative is also natural in the healthy spine. For example, rotational movement can be described as the clockwise or counter-clockwise twisting rotation of the vertebrae during a golf swing.
In a healthy spine, the inter-vertebral spacing between neighboring vertebrae is maintained by a compressible and somewhat elastic disc. The disc serves to allow the spine to move about the various axes of rotation and through the various arcs and movements required for normal mobility. The elasticity of the disc maintains spacing between the vertebrae during flexion and lateral bending of the spine thereby allowing room or clearance for compression of neighboring vertebrae. In addition, the disc allows relative rotation about the vertical axis of neighboring vertebrae allowing twisting of the shoulders relative to the hips and pelvis. A healthy disc further maintains clearance between neighboring vertebrae thereby enabling nerves from the spinal chord to extend out of the spine between neighboring vertebrae without being squeezed or impinged by the vertebrae.
In situations where a disc is not functioning properly, the inter-vertebral disc tends to compress thereby reducing inter-vertebral spacing and exerting pressure on nerves extending from the spinal cord. Various other types of nerve problems may be experienced in the spine, such as exiting nerve root compression in the neural foramen, passing nerve root compression, and enervated annulus (where nerves grow into a cracked/compromised annulus, causing pain every time the disc/annulus is compressed), as examples. Many medical procedures have been devised to alleviate such nerve compression and the pain that results from nerve pressure. Many of these procedures revolve around attempts to prevent the vertebrae from moving too close to each in order to maintain space for the nerves to exit without being impinged upon by movements of the spine.
In one such procedure, screws are embedded in adjacent vertebrae pedicles and rigid rods or plates are then secured between the screws. In such a situation, the pedicle screws press against the rigid spacer which serves to distract the degenerated disc space thereby maintaining adequate separation between the neighboring vertebrae to prevent the vertebrae from compressing the nerves. Although the foregoing procedure prevents nerve pressure due to extension of the spine, when the patient then tries to bend forward (putting the spine in flexion), the posterior portions of at least two vertebrae are effectively held together. Furthermore, the lateral bending or rotational movement between the affected vertebrae is significantly reduced, due to the rigid connection of the spacers. Overall movement of the spine is reduced as more vertebras are distracted by such rigid spacers. This type of spacer not only limits the patient's movements, but also places additional stress on other portions of the spine, such as adjacent vertebrae without spacers, often leading to further complications at a later date.
In other procedures, dynamic fixation devices are used. However, conventional dynamic fixation devices do not facilitate lateral bending and rotational movement with respect to the fixated discs. This can cause further pressure on the neighboring discs during these types of movements, which over time may cause additional problems in the neighboring discs.
Accordingly, dynamic systems which approximate and enable a fuller range of motion while providing stabilization of a spine are needed.